From the Royal Observatory Brussels
1) Project: Numerical study of the chemo-dynamical
evolution of galaxies
This project is articulated around two main research orientations:
(1) the chemo-dynamic evolution of galaxies and (2) the modelling
of the stellar thermonuclear combustion associated with explosive
astrophysical phenomena (thermonuclear supernovae, thermonuclear
burning at the surface of accreting compact stars).
The axis "modelling the chemical and dynamic evolution of galaxies",
started by H. Boffin at the ROB, aims at coupling the dynamic and
chemical properties (stellar nucleosynthesis) of the gas to the
stellar component of a galaxy. In practice, we develop a Nbody +
hydrocode based on the Lagrangian TREE-SPH method. After a first
phase (November 2002 - April 2003) of examining the possibilities
offered by the SPH code of Boffin, it was decided to adopt the code
"GADGET" (GAlaxy with Dark matter and Gas IntEracT) available from
the authors at the Max-Planck Institut für Astrophysik in
Garching-bei-München. The analysis of this code was undertaken in
order to integrate the cooling of the gas as well as a star formation
algorithm. The first application is the modelling of a prototypical
spiral galaxy (the Milky Way) and, to that purpose, we have undertaken
the simulation of a series of numerical test cases presented in the
paper of Carraro et al (1998, MNRAS 297, 1021). This project relies
on collaborations with N. Prantzos (IAP, Paris) and S. Boissier
(Carnegie Obs. Pasadena, USA) and with A. El Messoudi (PhD student
of M. Arnould at ULB).
The axis "modelling of the stellar thermonuclear combustion" of the
project concerns the study of the ignition and propagation phases of
an explosive thermonuclear combustion in degenerate and relativistic
stellar plasma. The ultimate goal of this research is the study of
the nucleosynthesis associated with various cataclysmic stellar events
(SNIa, Classical Novae). A concomitant objective is the development of
numerical methods that can contribute in the future to the development
of multidimensional (3D) stellar hydrodynamics. This research orientation
is primarily multidisciplinary, as emphasized by the collaborations.
Experts in terrestrial combustions will collaborate with astrophysicists
on the study of astrophysical situations. In this context, links have
been successfully established with non-astrophysical scientific communities,
international as well as national: Service d'Astrophysique du Commissariat
à l'Energie Atomique (CEA), Paris ; Laboratoire de Combustion et de
Détonique, Poitiers ; Laboratoire de Mathématique
Appliquée , Université
de Bordeaux ; Unité de Thermodynamique de la Faculté Polytechnique,
Université Catholique de Louvain). On the scientific part, Dr. Busegnies
devoted the period 2002-2003 to the development of (a) an equation of
state appropriate to the exploding stellar plasma, (b) an adequate Riemann
Solver for this equation of state. This activity falls in line with the
prospective study of a cellular thermonuclear detonation likely to
develop at the surface, or in the internal part, of a white dwarf star.
Publications from the ORB/KSB
"SPH simulations of reactive shock tubes in an astrophysical perspective".
Busegnies Y, Francois J., Paulus G.
Submitted to
Journal of Computational Physics, July 2003; revised version: December 2003
2) Project: Fundamental stellar parameters from
binaries and young stellar groups
Binaries are an important source of precise fundamental stellar
parameters and hence provide empirical constraints on stellar evolution.
In stellar groups, they provide anchor points for the interpretation
of the whole stellar population. Our investigation concentrates on
young stellar groups and well-detached binaries, but not exclusively
since binaries in fast phases of stellar evolution and specific single
stars have also a considerable potential in stellar evolution tests.
In particular, the technique of spectral disentangling, which
reconstructs the individual component spectra, is applied to
double-lined (eclipsing) binaries in order to obtain fundamental
stellar parameters of high accuracy. The stars and systems under
study include:
RV Crateris, an eclipsing pre-main sequence triple, of which 59
high-resolution spectra over the whole orbital cycle of the close
binary were obtained in 2003 in a cooperation with colleagues from
UFMG, Belo Horizonte. Preliminary results include the radial
velocity (RV) curves for the close binary and the spectral-type
identification of the components (F, G and K) from disentangled
parts of spectra.
Muscae, an eclipsing SB2 in Sco-Cen
accompanied in a wider orbit by a less massive pre-main sequence
component and further including a chemical peculiar (CP), magnetic
common proper motion star. The orbital period of the eclipsing pair
consisting of two B-type stars was shown to be 57.5 h, rather than
the 20 d value claimed in the literature, the two eclipses were for
the first time scanned in detail with uvby photometry at the SAT,
and high-resolution spectra were obtained in Chile and South Africa.
HD123335, also an eclipsing SB2 in Sco-Cen, was found to be
in a very eccentric orbit of 35 d, from 100 nights of uvby photometry
at the SAT in 2003, supplemented by unpublished photometry from
Balona at SAAO, and high-resolution spectroscopy from Chile, South
Africa and New Zealand in 2002-2003. The secondary turned out to be
a very slowly rotating He-weak SrTi B-type magnetic star with a rotation
period of 2 months. Observing time has been obtained to exploit the
unique situation that the surface of this peculiar star can be
scanned during eclipse.
NGC224-#334 is a very young (single) B-type CP star in the
Rosette Nebula, which we detected a few years ago. In collaboration
with the group of Landstreet (Univ. Western Ontario) this 13th
magnitude star was shown to possess a huge magnetic field and pronounced
peculiar abundances, using FORS and UVES observations. Being the
youngest known star of its class, it puts a strong empirical constraint
to the time scale needed to develop the overabundances, which may
turn out to be incompatible with the (slow) process of diffusion in
the stellar atmosphere.
CPD-59°2628, an eclipsing, fast-rotating, massive (O9.5+B0)
SB2 in the young open cluster Tr 16, was first analysed and results
were published in 2001. Since then, additional spectroscopy was
obtained and now cooperation with Keele Univ. (UK) will extend the
data set further, making it suitable for an application of the spectral
disentangling technique.
GL Carinae, another fast-rotating early-B SB2 in an eccentric
orbit, shows fast apsidal motion. The collaborators in Copenhagen (CUAO)
have separately made a full photometric analysis. Their spectra are
currently reduced with our improved FEROS software and prepared for a
spectral disentangling analysis.
AC Velorum, a hierarchical triple with an eclipsing close binary
consisting of two similar, evolved, B-type stars of 6.9 and
7.1 M
, has the potential to
constrain the occurrence of mixing of processed material into the
stellar atmosphere. The study of the light-time effect in the photometry
revealed that the fainter third object consists itself of at least two
stars with a combined mass of more than
6 M. The first spectroscopic
evidence for the third object was found as an extra contribution
in the Balmer lines. Out of the Balmer lines, the spectra of the
components of the close binary have been disentangled and are used
to derive atmospheric parameters and abundances.
HD 73256, a late-type star, was found by the CORALIE
planet-search programme to be orbited by a 1.85 Jupiter-mass
planet. A search for a planet transit with SAT in February 2003
gave a negative result, but did instead reveal the rotation
period of the star.
In several of these analyses and observational efforts, two IAP
partners are involved and collaborators in Antwerp, Copenhagen,
Zagreb or Belo Horizonte. In particular the important contribution
of the extensive photometry at the SAT would not have been possible
without the coordinated financial and scientific effort of the
two IAP partners in Brussels.
Finally, the spectra of the recurrent nova IM Normae which were
obtained with FEROS in 2002 contributed to show (Duerbeck,
Sterken, Baptista et al. 2003) that IM Nor is a member of the
class of hybrid novae.
Publications in 2003 from the ORB/KSB and VUB
Spectroscopic and photometric observations of the recurrent nova IM Nor
Duerbeck, H.W., Sterken, C., Baptista, R., Cunha, K., Diaz, M.P., Dutra, C.M., Freyhammer, L.M., Hensberge, H., Jones, A.F.
in "Interplay of Periodic, Cyclic and Stochastic Variability in Selected Areas of the H-R Diagram" (eds. Christiaan Sterken), ASP Conf. Ser. 292, 309.
Muscae, a multiple system with a PMS component
Hensberge, H., Nitschelm, C., Bouzid, M.Y., Clausen, J.V., David, M., Freyhammer, L.M., Helt, B.E., Olsen, E.H., Sterken, C., Vaz, L.P.R.
in: "Open issues in local star formation" (eds. Jacques Lépine & Jane Gregorio-Hetem), ASSL 299, CD-Rom (ISBN 1-4020-1755-3)
Publications in 2003 from the ORB/KSB
The CORALIE survey for southern extra-solar planets. X. A Hot Jupiter orbiting HD 73256
Udry, S., Mayor, M., Clausen, J.V., Freyhammer, L.M., Helt, B.E., Lovis, C., Naef, D., Olsen, E.H., Pepe, F., Queloz, D. & Santos, N.C.
A&A 407, 679
From K.U. Leuven
1) Research carried out by Evelien Vanhollebeke
Evelien Vanhollebeke's Ph.D. thesis is on the subject of
``The stellar population of the inner Galaxy'' and started
October 1st, 2003.
This will be in part a theoretical study and partly an
observational one. The theoretical part will involve the
use of a ``galactic model'' to predict number and colours
of stars along a few low extinction lines of sight towards
the Galactic Bulge. The confrontation with observational
data, in particular the near-infrared DENIS and 2MASS
surveys, the mid-infrared ISOGAL survey, and the OGLE
I-band data, which also provides information on the
variable nature of the sources, will allow to constrain
the main characteristics of the Bulge: orientation, extent,
number density, and star formation. After the initial
calibration of this model, it can be used to predict
numbers and arbitrary colours along any Bulge line of sight.
The observational part of her thesis involves the metallicity
of AGB stars in the bulge as derived from near-infrared
spectra, and analysis of the dust content of Bulge AGB stars
from mid-infrared spectra that will be obtained from a
recently submitted SPITZER proposal.
In the period October - December 2003 Evelien has reduced
SOFI H,K band spectroscopy of a sample of Galactic Centre
OH/IR stars. Kinematical studies had shown that this sample
consisted of groups of stars with different formation
histories. It is expected that the stars of the different
groups have different metallicities. It is the aim of this
program to prove this observationally. The data is reduced
and the analysis of the spectra is ongoing in collaboration
with people of the Observatoire de Besancon, Strasbourg
and the Institute for Astronomy in Vienna.
In January and February 2004, Evelien has been involved in
the preparation of a proposal for the SPITZER infrared
satellite. The aim of this project is to study the dust
content of a group of bulge AGB stars of 1Msun. We asked
for low-resolution IRS spectroscopy in the wavelength
range of 5.2 - 38 microns.
2) Research carried out by Greet Decin, K.U.Leuven
Study of the occurrence and time evolution of circumstellar
debris disks and their relation with planetary systems.
During 2003 Greet Decin has finished her PhD thesis, which was
successfully defended on November 12, 2003. In the last phases
of this work, she has elaborated on her earlier results and
finalized an important amount of new developments, all related
to the study of debris disks around normal stars.
During 2003 it has been found that a planet is present around
one of the four G dwarfs for which she detected and studied
the infrared signature of a debris disk in 2000. This is the
second known system where a detectable debris disk and a planet coexist.
The detection by Greet Decin of a similar fraction of disk
debris stars among G dwarfs as among A stars is in clear
contradiction with studies which claim to show a decrease of
the Vega phenomenon with stellar age. Consequently, Greet
Decin has carried out a broad and careful study of this age
dependence. It appears that several studies in the literature
are flawed for two reasons: first, the found decrease of the
phenomenon with age heavily depends on the inclusion in these
studies of young clusters, where the circumstellar disk represents
the remnant of the star formation process rather than a reprocessed
disk which a debris disk such as found around Vega is in reality;
second, a careful analysis of the age determinations and the
true excesses reveals that for several objects the literature
results are questionable. The resulting picture is one in which
debris disks occur throughout the full age range, a consistent
maximal fractional luminosity (IR over stellar) of the order of
0.1% is found, and a decrease is found with age for the lowest
observed excesses at a given age. Presently it is not clear
whether the latter effect represents a true evolutionary effect
or is a statistical fluke related with the low occurrence of
young stars in the solar neighbourhood.
In collaboration with Dr. Carsten Dominik (University of Amsterdam)
Greet Decin has carried out a theoretical study of the time
dependence of the Vega phenomenon, considering the various known
mechanisms which lead to the formation and decay of debris disks.
It was found that the lifetime of a debris disk does not depend
critically on its mass, but rather on the collision rate and the
Poynting-Robertson effect. The large fraction of G dwarfs with
debris disks can only partially be explained by the lower radiation
pressure of these cooler stars.
In this same context, the fairly high claimed occurrence of debris
disks around K giants is puzzling. From a careful analysis of
such objects with IRAS and ISOPHOT data it was found that in all
stars except one the observed IR excesses are extended. This
means that the excesses are not genuine, but due to the occurrence
of the star in the vicinity of an interstellar cirrus cloud.
For the remaining object the debris disk appears to be real.
An abundance analysis has been carried out of the photospheres
of stars with debris disks. Where for stars harbouring a planet
the typical metallicity is larger than average, this is not the
case for debris disk stars. While both distributions overlap,
allowing for the existence of objects showing both phenomena,
this result suggests that debris disk stars follow a regime which
is genuinely different from that of stars with planetary systems.
Publications in 2003 from the KU Leuven
ISO-SWS calibration and the accurate modelling of cool-star atmospheres. IV. G9 to M2 stars
Decin, L., Vandenbussche, B., Waelkens, C., Decin, G., Eriksson, K., Gustafsson, B., Plez, B., Sauval, A. J.
Astronomy and Astrophysics, 400, 709-727
Age Dependence of the Vega Phenomenon: Theory
Dominik, C., Decin, G.
Astrophysical Journal, 598, 626-635
Age Dependence of the Vega Phenomenon: Observations
Decin, G., Dominik, C., Waters, L. B. F. M., Waelkens, C.
Astrophysical Journal, 598, 636-644
